I agree with all the savvy ladies who have posted before me. But I did want to add my own personal experience.

I haven't had a period in well over 2 years now - maybe longer. I can't even remember the last time I bought tampons or pads. Every OB/GYN I have been to in this period (3 different ones in 3 different cities trained at 3 different schools) have never been concerned. What they told me is that with my activity level and the way I eat (I'm thin but I definitely have to be a t least 20% body fat... there's some squish there) that I have neither growth (increased endometrial tissue) nor shedding each month as my hormones fluctuate.

I'm not a human doctor (dogs/cats/llamas/what have you are totally different from primates!!!) but a second opinion couldn't hurt. In my own life, I've never been told, even though I've asked, that this is going to be harmful to me later in life. They always say it's just my unique set of hormones and body chemistry.